Generally, a wireless communication system supports an active mode of a Mobile Station (MS) and an idle mode thereof. The active mode is a state in which the MS is performing communication. The idle mode is a state in which the MS stops communication due to the lack of transmitted/received traffic and identifies only the generation or non-generation of traffic. A location of the MS of the active mode is recognized in the cell unit, but a location of the MS of the idle mode is recognized in the area unit including at least one cell. The area unit is designated as a paging area, a Tracking Area (TA) and the like.
One tracking area includes at least one cell and, upon network design, division of the tracking area is defined. Each Base Station (BS) knows to which tracking area itself belongs, and broadcasts information notifying a tracking area that itself belongs to. According to this, through the information transmitted in the BS, an MS can identify in which tracking area itself is now located. And then, the MS informs a system in which tracking area itself is located. This procedure is designated as Tracking Area Update (TAU). The tracking area update procedure is carried out when there is a change of a tracking area where the MS is located. Due to this, the system can track in which tracking area the MS is now located.
However, in a case where the changed tracking area has been included in a Tracking Area List (TAL) of the MS, the MS does not perform the tracking area update procedure. The tracking area list includes at least one tracking area where the MS is now located or was located in the past. The tracking area list is a range in which paging is performed. If traffic going toward the MS is generated, the system transmits a message of paging the MS using at least one BS that is installed in at least one tracking area included in the tracking area list of the MS. According to this, the MS recognizes the generation of the traffic to be received, and transits to the active mode.
For the tracking area update procedure, an MS transmits a request for uplink resources through a Random Access CHannel (RACH), and transmits a tracking area update request message through the uplink resources. At this time, when attempts on tracking area update request procedures are made by a plurality of MSs in a specific tracking area, a computational load of the system is intensively increased. Further, because the plurality of MSs transmit signals through the random access channel, congestion in the random access channel occurs and thus, a possibility of a signal transmission failure increases. If a failure of the random access channel occurs, tracking area update cannot be carried out and, as a result, a possibility of a paging failure increases.